1. Field of the Invention
The present invention relates to an optical module and a scan-type image projection display device. In particular, the present invention relates to, for example, an optical module for coupling optical beams emitted from a plurality of laser diodes into a beam on one optical axis and outputting the coupled optical beam, and a scan-type image projection display device for displaying images by use of the optical beam from the optical module.
2. Description of the Related Art
Small-sized projectors, easily portable and capable of displaying images in large screen sizes, are being developed actively in recent years. Small-sized projectors connectable to notebook PCs, video cameras equipped with a built-in projector capable of projecting recorded images, etc. are already available commercially. Cellular phones and smartphones equipped with a built-in projector are also expected to appear in the near future.
As the projecting methods for the projectors, the type is known in which a lamp or an LED (Light Emitting Diode) is used as the light source and images displayed by liquid crystal panels or a DMD (Digital Micromirror Device) is projected. On the other hand, laser projectors (scan-type image projection display devices) are also under development. Such projectors use a laser diode as the light source and display images by allowing a movable mirror to scan an optical beam emitted from the laser diode. Since a laser diode emitting a laser beam is used as the light source, the focusing is unnecessary and the luminance of the images can be increased with ease. Therefore, the laser projectors (scan-type image projection display device) are considered to be highly suitable for the purpose of projecting images onto an appropriate wall available at a place away from home.
Further, the laser projectors (scan-type image projection display device) may be installed in cars while taking advantage of the high luminance of images. In this case, it is conceivable that the laser projectors will be applied to head-up displays for projecting and displaying on the windshield images such as navigation images.
The configuration of a scan-type image projection display device capable of displaying color images by use of laser diodes of three colors (red, blue, green) has been described in JP-T-2009-533715, for example. The scan-type image projection display device of the JP-T-2009-533715 includes a beam coupling unit which couples the three laser beams of the three colors into a synthetic beam propagating along one optical axis and a beam scanner which scans (periodically changes) the deflection direction of the synthetic beam. In the configuration of the beam coupling unit described in the JP-T-2009-533715, the three light sources are arranged in parallel. The three beams from the light sources are emitted in the same direction, reflected by corresponding beam coupling mirrors, respectively, and thereby coupled into the synthetic beam.
Incidentally, while green light is acquired by performing wavelength conversion on infrared light by means of SHG (Second Harmonic Generation) also in the JP-T-2009-533715 since there were no laser diodes directly emitting green light, such laser diodes directly emitting green light are becoming available in these years.